A light emitting device refers to an inorganic semiconductor device which emits light through recombination of electrons and holes, and has been recently applied to various fields such as displays, vehicle lamps, or general lighting, optical communication, and the like. Particularly, application ranges of the UV light emitting device have been broadened to UV curing, sterilization, white light sources, medicine, assistant components, and the like.
A light emitting device generally comprises an n-type semiconductor layer, a p-type semiconductor layer, and an active layer formed therebetween. Since the UV light emitting device emits light having a relatively short peak wavelength (generally, light having a peak wavelength of 400 nm or less), AlGaN containing 20% or more of Al is used when the UV light emitting device is fabricated using a nitride-based semiconductor. When band-gap energy of n-type and p-type nitride semiconductor layers of the UV light emitting device is smaller than energy of UV light, UV light emitted from the active layer can be absorbed into the n-type and p-type nitride semiconductor layers in the light emitting device. In this case, the light emitting device has significantly deteriorated luminous efficacy. Accordingly, not only the active layer of the UV light emitting device, but also other semiconductor layers disposed on a light emitting direction of the light emitting device contain 20% or more of Al.
In fabrication of the UV light emitting device, a sapphire substrate is used as a growth substrate. However, when an AlxGa(1-x)N (0.2≦x≦1) layer is grown on the sapphire substrate, the AlxGa(1-x)N (0.2≦x≦1) layer can suffer from cracking or breaking due to thermal or structural deformation resulting from high Al composition. These phenomena result from lattice mismatch and/or difference in coefficient of thermal expansion between the sapphire substrate and the AlxGa(1-x)N (0.2≦x≦1) layer. In the prior art, to prevent such problems in fabrication of the light emitting device, an AlN layer is grown on the sapphire substrate at high temperature, or an AlN/AlGaN super-lattice layer is formed on the sapphire substrate, followed by forming an N-type semiconductor layer comprising AlxGa(1-x)N (0.2≦x≦1), an active layer, and a P-type semiconductor layer.